To form a wiring layer, an electrode, and the like of an electronic apparatus, silver pastes, such as a resin type silver paste and a baked type silver paste, have been widely used. In other words, these silver pastes are applied to or printed on various kinds of substrates, and then heat-cured or heat-baked, whereby a conductive film to serve as a wiring layer, an electrode, or the like can be formed.
For example, the resin type silver paste comprises a silver powder, a resin, a curing agent, a solvent, and the like, and is printed on a conductor circuit pattern or on a terminal, and heat-cured at a temperature of 100° C. to 200° C. to be made into a conductive film, whereby wiring or an electrode is formed. On the other hand, the baked type silver paste comprises a silver powder, glass, a solvent, and the like, and is printed on a conductor circuit pattern or on a terminal, and heat-baked at a temperature of 600° C. to 800° C. to be made into a conductive film, whereby wiring or an electrode is formed. In the wiring and the electrodes formed of these silver pastes, an electric current path in which an electrical connection is established by linkages between the silver powders is formed.
The silver powder to be used for the silver paste has a particle diameter of 0.1 μm to a few μm, and the particle diameter of the silver powder to be used differs depending on the thickness of a wiring or an electrode to be formed. Furthermore, uniform dispersion of the silver powder in the paste enables the formation of wiring having a uniform thickness or the formation of an electrode having a uniform thickness.
Generally, a silver paste is produced in such a manner that, for example, first, a silver powder is preliminarily kneaded smoothly with other components such as a solvent, and then kneaded with a predetermined pressure being applied using a three-roll mill or the like. This manner enables a large amount of a silver paste to be produced at a time, whereby higher productivity and an effect of production cost reduction can be expected. Meanwhile, a silver powder is required to be efficiently kneaded by a roller, in other words, required to have good kneadability.
A paste having a too high or too low viscosity is difficult to efficiently knead by a three-roll mill. A silver powder having a low viscosity causes a shearing stress in a three-roll mill to be smaller and causes a shearing force applied to a silver paste to be smaller, and accordingly the silver powder is insufficiently dispersed in the paste. On the other hand, a silver powder having a high viscosity is difficult to knead smoothly with other components such as a solvent, and consequently, a paste obtained by insufficiently kneading the silver powder with other components such as a solvent is fed into a three-roll mill.
In the case where a silver powder is insufficiently dispersed in a paste, or in the case where the viscosity of a paste is decreased due to insufficient kneading of a silver powder with other components such as a solvent, aggregate masses of silver particles are present in the paste. When such paste is kneaded by a three-roll mill, the aggregate masses of the silver particles are crushed, whereby a coarse powder such as a powder in the form of a thin piece having a few mm size (flakes) is generated. It is not desirable to leave the generated flakes in the paste as they are, and therefore the flakes are sieved using a mesh or the like and removed. Too much amount of flakes generated cause troubles such as clogging of the mesh with the coarse powder, whereby the flakes cannot be efficiently removed, and productivity is considerably reduced.
Furthermore, in the case where flakes occur in a paste as mentioned above, when screen-printing is conducted using that paste, a fine screen is clogged up with the coarse flakes, thereby causing difficulties in accurate printing of a pattern.
Thus, the flake occurrence at the time of paste production has a great impact on printability in screen-printing. Therefore, it is desired that a silver powder has a viscosity enough to be easily kneaded at the time of paste production and has good dispersibility in a solvent, and also has strength enough not to cause masses of silver particles to be crushed during the kneading.
To make paste production easier, the control of the particle size distribution of a silver powder and the form thereof has been proposed.
For example, Patent Literature 1 proposes a conductive adhesive agent in which 30% to 98% by weight of a silver powder is blended as a conductive powder with a resin for a binder. The silver powder whose primary particles have a flat shape and which has a massive aggregation structure having a tap density of not more than 1.5 g/cm3 is contained in the conductive adhesive agent.
According to Patent Literature 1, the silver powder having the aggregation structure has high dispersibility enough to be easily released from aggregation and changed into primary particles, and, without causing the deterioration of conductivity due to poor dispersion of the silver powder, stable electrical conductivity can be generated, and there can be obtained a conductive adhesive agent which realizes a curing material excellent in not only conductivity but also adhesiveness, heat resistance, moisture resistance, workability, and thermal conductivity.
However, this proposal does not consider the occurrence of coarse flakes caused by a change in the viscosity of a paste or re-aggregation of silver particles dispersed in the paste, and hence it is difficult to say that the dispersibility in a finally-obtained paste is secured.
Patent Literature 2 proposes the addition of a nonionic surface active agent having an HLB value of 6 to 17 to a silver-complex-containing solution. This aims to prevent the aggregation of reduced silver particles at the time of the addition of a reducing agent. Patent Literature 2 proposes a method for producing a silver powder wherein the addition rate of a solution containing a reducing agent is made higher, that is, not less than 1 equivalent per minute, whereby there is obtained a silver powder having a tap density of not less than 2.5 g/cm3, a mean particle diameter of 1 to 6 μm, and a specific surface area of not more than 5 m2/g, and being excellent in dispersibility.
However, this proposal is to prevent the aggregation of an obtained silver powder, thereby achieving a dispersed silver powder, but, does not consider the dispersibility of a silver powder in a solvent and the occurrence of flakes at the time of paste production.
Patent Literature 3 proposes a conductive paste, comprising: conductive particles having a mean particle diameter of 0.5 to 20 μm and a specific surface area of 0.07 to 1.7 m2/g and containing aggregating particles having a degree of aggregation of 1.05 to 3.90 (the degree of aggregation=the mean diameter of the aggregating particles/the mean diameter of primary particles); and a binder containing a thermosetting resin as a main component.
According to this proposal, a conductive paste having good flowability and dispersibility is obtained, conductive particles are stably filled in a via and stably come in contact with each other inside the via hole, and high-quality via hole conductors can be stably formed with less variation.
However, this proposal aims at the filling property of the paste into a via and high connection-reliability, and does not consider the dispersibility of a silver powder itself in a solvent and the occurrence of flakes at the time of paste production. Furthermore, the proposal defines only the degree of aggregation, and does not define whether the degree of aggregation is based on aggregates (which are in face-contact and have high strength) or based on agglomerates (which are in point-contact and have weak strength), and thus does not consider the occurrence of flakes.
Patent Literature 4 defines that the mean particle diameter DIA of primary particles observed using a scanning electron microscope is not more than 0.6 μm and the ratio D50/DIA is not more than 1.5, where D50 is a mean particle diameter determined by a particle-size-distribution measurement method using laser diffraction scattering, and thus Patent Literature 4 proposes a silver powder having excellent dispersibility. Such silver powder having excellent dispersibility could reduce the viscosity in a solvent at the time of paste production and cause the occurrence of flakes.
Patent Literature 5 defines that the D50/DIA is not more than 5. According to this definition, in the measurement of particle size distribution, the value (MT-D50) in (water+a surface active agent) is not less than 5 μm, and accordingly, flakes could occur in a solvent at the time of paste production.
As mentioned above, there have been proposed improvements in the dispersibility of a silver powder in a paste and the conductivity and reliability of an electrode and wiring which are formed using the paste. However, the prevention of flake occurrence at the time of paste production has not been proposed.